The present invention relates to a machine tool, and more particularly to a type thereof wherein a workpiece is easily mounted on an indexing table and the workpiece is easily visible during machining. The present invention also relates to an indexing apparatus wherein locking of an indexing table, unlocking of the table, and rotational indexing thereof can be performed successively by a single drive source, the table mounting thereon a workpiece.
There are various types of machine tools such as a drilling machine which forms bores in a workpiece, a tapping machine which forms internal thread on a previously formed bore, and a machining center (MC) which performs various kinds of cutting in automatically changing and selecting an optimum tool.
These machine tools are divided broadly into two categories in terms of arrangement or orientation of a main spindle which mounts a tool, that is, a vertical axis type as shown in FIG. 12 and a horizontal axis type as shown in FIG. 13. In the vertical axis type shown in FIG. 12, guide rails 12 extend vertically on a vertical guide face 10a of a column 10, and a main spindle head 14 is provided vertically movable along the guide rails 10a. A main spindle 16 is oriented downwardly from the spindle head 14. An indexing table 18 extends in horizontal direction at a position below the guide face 10a, and a workpiece 22 is detachably secured to the table 18 through a jig 20.
In the vertical axis type, a rotational axis c of the indexing table 18 is directed in horizontal direction and perpendicular to the main spindle 16. Therefore, operator must fix the jig 20 and the workpiece 22 to a vertically orienting fixing plane 18a of the indexing table 18, and the operator must replace the jig 20 and/or the workpiece 22 from another one at the vertically extending plane. Such fixing or replacing works would be rather troublesome. Particularly, if the workpiece 22 has heavy weight, the operator must lift the workpiece and abut the same on the vertically extending fixing plane 18a of the indexing table 18, which work requires hard labor.
With respect to the horizontal axis type shown in FIG. 13, guide rails 12 extend in horizontal direction on a horizontal bed 24, and a main spindle head 14 is disposed movable in horizontal direction along the guide rails 12. An indexing table 18 which holds a workpiece 22 is positioned in front of a main spindle 16. That is, if observing from the operator's side, the indexing table 18 is positioned frontward side of the machine tool, and the main spindle head 14 is positioned behind the table 18.
According to the horizontal axis type, the workpiece 22 is subjected to machining at the rear side of the table 18. Therefore, operator who is positioned in front of the machine tool cannot easily observe the machining condition of the workpiece. In order to visually observe the machining condition, the operator must walk around the machine tool, so that additional space is required for observation around the machine tool in addition to the machine installation space. Therefore, bulky space is required. Further, a tool fixed on the main spindle 16 moves in horizontal direction during machining. If hole is formed in the workpiece, the hole also extends horizontally, so that tool lubrication oil cannot be sufficiently reached to the tool blade portion. As a result, tool damage or thermal seizure thereof may occur due to insufficient circulation of the lubrication oil. One example of a conventional machine tool is disclosed in Japanese Utility Model Publication No. 61-6023, in which vertically extending jig mounts thereon a workpiece, and a machine head and a tool is positioned behind the workpiece.
Turning to the indexing apparatus, the machining center (MC) normally includes the index table which fixedly secures a workpiece. The index table serves as the indexing apparatus. The indexing apparatus generally includes a geneva mechanism for intermittent rotation of the table which mounts the workpiece in order to index one of the workpiece planes to be machined relative to the tool.
In the indexing apparatus used in a machine tool such as the machining center, heavy mechanical load is applied to the table and the workpiece mounted on the table during various machining to the workpiece. Therefore, a locking mechanism is provided for fixing the table to a main body of the machine tool so as to prevent the table from its rotation during machining. For example, Japanese Utility Model application Publication (Kokai) No. 60-29042 discloses a mechanism for locking a table 203 to a support base 201 during machining so as to receive heavy load on both the table 203 and the support base 201 as shown in FIG. 14. The locking is released when the rotational indexing is conducted for allowing the table to be rotated. More specifically, in the indexing apparatus shown in FIG. 14, coupling means 204 204 are provided between the table 203 and the support base 201. The coupling means are in selective meshing engagement with each other and the meshing can be disengageable from each other. The coupling is so called Curvic gears(Trade name). The table 203 is vertically movable by means of a power cylinder drive means 205. For the rotational indexing, the table 203 is lifted by the power cylinder 205 to disengage one curvice gear 204 from the other, and then the table 203 is rotated by a drive motor. Upon completion of the indexing, the power cylinder drives the table 203 downwardly to provide meshing engagement between the gears 204 204, so that the table 203 is in locking engagement with the support base 201 for withstanding heavy load by the entire apparatus during machining.
As described above, such conventional indexing apparatus positively locks the table to the main body during machining, and the table is lifted to release the locking for rotational indexing. For this, the conventional device requires additional driving source such as the above-described power cylinder means for vertically moving the table to fix and release the table in addition to the main drive source for rotationally indexing the table.
Such conventional indexing apparatus requires complicated structure and high production cost due to the provision of two drive sources. Further, it takes relatively long time for the rotational indexing and locking unlocking operation due to independent switching operations of the two drive sources.